Nowadays, there exist a variety of wireless communication systems, such as CDMA2000, WCDMA, WiMAX, Wi-Fi, and so on. A common problem that these wireless communication systems face is how to improve the transmission efficiency of the air interface. Therefore, in the physical layer (PHY) and layer 2 (such as MAC in LTE, MAC in UMB, MAC in WiMAX system, and MAC layer in Wi-Fi), how to obtain higher transmission efficiency with limited spectrum bandwidth and limited wireless resources is being considered.
The issue mainly considered in this specification is the design for the message format of layer 2.
Take IEEE802.16e-2005 for example, in which, at present, the message structure of DCD (Downlink Channel Description) supporting OFDMA is as follows:
SyntaxSizeCommentsDCD_Message_Format( ) {Management Message Type = 18 bitsReserved8 bitsShall be set to zeroConfiguration Change Count8 bitsTLV Encoded information forVariable lengthTLVthe overall channel(Type-Length-Value)formatBegin PHY Specific Section {See applicablefor (i = 1; i <= n; i++) {PHY chaptersDownlink_Burst_Profile }}}
Wherein, the content of the domain TLV Encoded information for the overall channel is defined as follows:
TypeName(1 byte)LengthValue (Variable length)PHY scopeDownlink_Burst_Profile1May appear more than once (see 7.1.2.3.1).AllLength is the number of bytes of all theobjects, including the embedded TLV items.BS EIRP22Signed, in units of 1 dBMAllFrame34The number of PSs contained in a Burst FDDSCdurationor TDD frame. Required only for frameddownlinks.PHY type41The PHY type to be usedSCPower510 = Preserve peak powerSC, SCaadjustment1 = Preserve Mean PowerruleDescribes the power adjustment rule whenperforming a transition from one burst profileto anotherChannel Nr61Downlink channel number as defined inSCa,802.16-2004 8.5. Used for license-exemptOFDM,operation only.OFDMATTG71TTG (in units of PS).SCa,Only used in TDD systemsOFDMARTG81RTG (in units of PS)SCa,Only used in TDD systemOFDMAEIRxPIR, max92Maximum isotropic received power of InitialAllRanging at BS. Signed, in units of 1 dBmChannel103Channel switch frame number as defined inSCa,Switch Frame802.16-2004.8.5. Used for license-exemptOFDM,Numberoperation only.OFDMAFrequency124Downlink central frequency (kHz)AllBSID136BSIDSCa,OFDM,OFDMAFrame141The duration of the frame.OFDMDurationCodeFrame153The number of the frame containing the DCDOFDMNumbermessage. If the DCD is fragmented, thisparameter is the frame number of the lastfragment of the DCD message.H-ARQ ACK1711 = 1 frame offsetOFDMAdelay for UL2 = 2 frame offsetburst3 = 3 frame offsetPermutation1910 = PUSCOFDMAtype for1 = FUSCbroadcast2 = optional FUSCregion in3 = AMCHARQ zoneMaximum201Maximum number of retransmission inOFDMAretransmissionDL HARQ. Default value shall be 4retransmissions.Default RSSI211Bit#0-3: Default averaging parameter avg forOFDMAand CINRphysical CINR measurements, in multiples ofaveraging1/16 (range [1/16, 16/16], 0x0 for 1/16, 0Xfparameterfor 16/16).Bit #4/7: RSSI measurements multiple (range[1/16, 16/16], default value is defaultaveraging parameter avg for 0x3, 1/16 of1/16, 0xF for 16/16)..Downlink226A bitmap describing the physical bands isOFDMAAMCallocated to the segment in downlink, whenallocatedallocating AMC subchannels through thephysicalHARQ MAP, or through the Normal MAP, orbands bitmapfor Band-AMC CINR reports, or using theoptional AMC permutation. The LSB of thefirst byte shall correspond to band 0. For anybit that is not set, the corresponding band shallnot be used by the SS on that segment. Whenthis TLV value is not present, BS may allocateany physical bands to a SS.Downlink34variableNum_region (6 bits for the number of regions,region2 bit reserved)definitionFor (i = 0, i < Num_region; i ++){OFDMA symbol offset (8 bits)Subchannel offset (6 bits)Number of OFDMA symbols (8 bits)Number of subchannels (6 bits)}Padding bitsHandoff type501Bit 0: HOOFDMAsupportedBit 1: MDHOBit 2: FBSS HOBit 3-7: ReservedH_Add311Threshold used by the MS to add a neighborOFDMAThresholdBS to the diversity set. When the CINR of aneighbor BS is higher than H_Add, the MSshould send MOB_MSHO-REQ to requestadding this neighbor BS to the diversity set.This threshold is used for the MS to performMDHO/FBSS HO. This value is in the unit ofdB. If the BS does not support FBSSHO/MDHO, this value is not set.H_Delete321Used by the MS to delete a BS from theOFDMAThresholddiversity set. When the CINR of a BS is lowerthan H_Delete, the MS should sendMOB_MSHO-REQ to request deleting thisBS from the diversity set. This threshold isused for the MS to perform MDHO/FBSSHO. This value is in the unit of dB. If the BSdoes not support FBSS HO/MDHO, this valueis not set.ASR (Anchor331Bit #0-#3: M, in units of framesOFDMASwitchBit #4-#7: L, in units of ASR slotsReport) SlotLength (M)and SwitchingPeriod (L)Paging Group352One or more logical affiliation grouping of BSIDTUSC1369When using the TUSC1 permutation, thispermutationbitmap describes the subchannels allocated toactivethe segment in downlink. The LSB of the firstsubchannelsbyte shall correspond to subchannel 0. For anybitmapbit that is not set, the MS on that segmentshall not use the corresponding subchannel.The active subchannels are renumberedconsecutively starting from 0.TUSC23713When using the TUSC2 permutation, thispermutationbitmap describes the subchannels allocated toactivethe segment in downlink. The LSB of the firstsubchannelsbyte shall correspond to subchannel 0. For anybitmapbit that is not set, the MS on that segmentshall not use the corresponding subchannel.The active subchannels are renumberedconsecutively starting from 0.Hysteresis511Hysteresis margin is used by the MS toAllmargininclude a neighbor BS to a list of possibletarget BSs. When the CINR of a neighbor BSis larger than the sum of the CINR of thecurrent serving BS and the hysteresis marginfor the time-to-trigger duration, the neighborBS is included in the list of possible targetBSs through MOB_MSHO_REQ message.This value is in the unit of dB and applies toHHO.Time-to-Trigger521Time-to-Trigger duration is the time durationAlldurationfor MS decides to select a neighbor BS as apossible target BS. This value is the unit of msand only applies to HHOTrigger54VariableThe Trigger is a compound TLV value thatlengthindicates the triggering mechanism. Thetrigger with this encoding is defined forserving BS or commonly applied neighborBSsN + I601The operator shall calculate and define theOFDMN + I (Noise + Interference) based on thecorresponding RF system designDownlink_burst_profile1531May appear more than once (see 7.1.2.3.1).OFDMAforThis length is the number of bytes in allmultiple FECobjects, including embedded TLV itemstypesRestart Count1541This value shall be increased by 1 wheneverAllBS restarts. This value increases from 0 to 255MAC Version1481See 8.1.3.2All
It can be seen that each of the above information units is constructed with the form of Type→Length→Value, that is to say, in order to carry a information unit, firstly, a byte should be used to describe the type of the information unit (Type), then a description of the length of the unit (Length), and the next is the genuine content of the information unit (Value).
This message is periodically broadcasted to all the terminals and wireless relay stations within the dominative region, and the terminals and wireless relay stations receive the message, identify the information within the message, and update the record of physical layer attributes. Through calculation, it can be known that the length of the message is at least 72 bytes. Some information segments in the message might be multiple, thus the length is more than 72 bytes in practical applications. According to the definition in 16e, the message is sent by broadcast, and sent at least once in every 10 s, and the 72 bytes needs to occupy at least 12 time slots, and based on a calculation under the condition that each time slot needs to occupy two symbols, one subchannel (24 data sub-carriers), and the uplink to downlink ratio is 1:1, the 72 bytes needs to be transmitted on a bandwidth of 0.41856M out of a total 10M bandwidth, that is, the message at least occupies 4.19% of the wireless resources during transmission (within 5 ms).
The message is just one of the MAC broadcast messages. According to 16e protocol, messages such as UCD, DLMAP and ULMAP are all sent by broadcast. It can be seen that these broadcast messages consume a lot of air interface wireless resources.
In other wireless systems, there are also similar broadcast messages transmitting physical-layer attributes of the channel, and this type of messages are generally periodically sent. So if the number of bytes occupied by this type of messages can be reduced, the occupancy of the wireless resources can be greatly saved.
In addition, due to the large occupancy of the wireless resources, the extension of the base station capability is almost impossible, for the reason that the capability extension needs more bytes to transmit the capability information, and if the amount of bytes occupied by such type of messages can be reduced, more capability selections can be provided under the same resource occupation, thus users can be provided with more personalized service.